DESCRIPTION (Applicant's abstract): A long range goal of psychobiology is to understand how internal physiological processes and external environmental factors interact to influence neural function and individual behavior. Improving our understanding of this interaction has strong implications for basic and applied human behavioral biology. The objective of this application is to examine and differentiate social and gonadal hormone influences on the function of the arginine vasotocin/vasopressin (AVT/AVP) system in the brain. The central hypotheses to be tested are: 1) that AVT expression is primarily influenced by the social environment in an animal model system where behavioral sex is determined by dominance interactions rather than the gonads, and 2) that AVT stimulates dominant male-typical sexual and offensive aggressive behavior through central actions in the brain of this teleost fish. The rationale behind this research is that while we know much about how gonadal hormones influence the brain and behavior, much less is known about potential direct social influences on the brain. The AVT system is a logical focus for asking how social influences act on the brain since this family of protein hormones has been strongly linked to the display of sexual behavior in all major groups of vertebrates and to aggression behavior in important mammalian models. The subject of this study, the bluehead wrasse (Thalassoma bifasciatum) undergoes complete behavioral and gonadal female-to-male sex change as an adult animal. This sex change is accompanied by a four- fold increase in AVT-mRNA levels in the brain area which exerts primary control of sexual and aggressive behaviors. The AVT/AVP system is influenced strongly by gonadal steroid hormones in many species. Importantly, however, behavioral sex change can occur even in the absence of gonads in bluehead wrasses and is instead completely dependent on social environment. This species therefore appears to present a unique opportunity to directly examine social influences on this important neural system. To address the central hypotheses above, the PI will pursue three specific aims: 1) Use natural behavioral variation and experimental dissections of social and gonadal influences to examine the control of AVT-mRNA expression, 2) Determine whether AVT can induce sexual and aggressive behavior in this species and 3) Determine sites of AVT action in the wrasse brain by AVT receptor localization and cellular markers of neural activation. Direct influences of social environment on the expression and behavioral actions of AVT would have relevance to humans, where sexual and aggressive behaviors are also under environmental influences.